The introduction of third-generation mobile radio systems requires very precise compliance with power levels of transmitted signals. For example, the UMTS/WCDMA mobile radio standard demands control of the emitted transmission power in order that the plurality of users who are transmitting in one frequency band do not excessively interfere with one another. The power control which is carried out both by the base stations and by the mobile stations is based on a “soft transition” between two different signal levels during a transmission or a reception process, thus avoiding interference signals in the process. There is therefore a fundamental requirement on the one hand to comply with the spectral requirements of the respective mobile radio standard, and on the other hand to reduce any phase or amplitude error in the transmitted signal.
A monitoring range of 80 dB is specified with an accuracy of 1 dB for the third-generation UMTS/WCDMA mobile radio standard and the CDMA2000 standard. Both pure voltage-controlled amplifiers and programmable amplifiers whose gain can be adjusted in discrete values can be used for these requirements.
In a programmable amplifier (PGC Programmable Gain controlled Amplifier), amplifier stages with a high gain are normally suitably combined with further amplifier stages with low gain. This makes it possible to cover a wide gain range with adequate resolution and accuracy at the same time. In order, by way of example, to cover the required range of 80 dB gain with an accuracy of 1 dB, it is known for a gain block to be provided in a programmable amplifier, in each case having five or six 1 dB stages and a second gain block with 13 6 dB gain stages.
When the gain in an amplifier such as this is changed to be a multiple of 6 dB, that is to say for example from 5 to 6 dB, the second gain block with the 6 dB steps is activated, and its gain is increased or reduced by 6 dB. At the same time, the overall gain of the first block is changed appropriately in the opposite direction in 1 dB stages.
By way of example, the first gain block with the 1 dB stages switches from a gain of +5 dB to a gain of 0 dB, while the second gain block changes its gain from 0 dB to +6 dB. The splitting of the programmable amplifier into two gain blocks with different resolution and different gain advantageously makes it possible to reduce the space consumed and the power.
On the other hand, FIG. 4A illustrates one frequent problem with a known programmable amplifier. In the example, the gain of 6 dB in the programmable amplifier is reduced to 5 dB. The gain of 6 dB means that the second amplifier block in the programmable amplifier is active with one amplifier stage, while the first amplifier block with the 1 dB amplifier stages is switched off. In order to make a reduction to 5 dB, the first block with its amplifier stages is now activated, and is used with full gain. At the same time, the second amplifier block is switched off.
This leads to an excessive signal level in the region of the transition of the gain from +6 dB to +5 dB, as is illustrated in FIG. 4A. Such an excessive signal level leads to an amplitude error and a phase error in the signal to be transmitted, and thus increases the bit error rate. Furthermore, downstream circuits can also be damaged by the high input value.